Not sure what your stock tire size is. It's 195/55-15 on the G3s. Better selection at 205/50-15. The +2 size with the least amount of speed error is 205/40-17 which is what I run now. I'm considering 215/40-17 which is about a 1.1% speed error. To calculate yours, just use this calculator:

http://www.miata.net/garage/tirecalc.html